The present invention relates to the field of devices and methods for dispensing small volumes, such as nanoliters, of liquid using a positive displacement pump. One type of positive displacement pump, known as a “bellows pump” comprises a body with chamber enclosing a bellows wherein the space between the bellows and the chamber walls is intended to be filled with a liquid that is to be dispensed and the liquid is dispensed through an outlet by expansion of the bellows. In another type of bellows pump the liquid to be dispensed is inside the bellows and the liquid is dispensed by moving the free end of the bellows by means of an actuator-driven rod positioned between the free end of the bellows and a fixed part of the pump body. Another type of positive displacement pump is a syringe pump in which the displacement of the piston of a syringe inside a syringe cylinder causes fluid to be sucked into or pressed out of the syringe outlet.
U.S. Pat. No. 5,638,986 describes a bellows pump in which the dosing space is formed by the interior volume of a short bellows which is compressed and expanded along its longitudinal axis by a stretching and contracting rod made of piezo-material which is in direct contact with the centre of an end surface of the bellows. The rod can expand and contract in the longitudinal direction of the bellows and acts on the centre of the bellows. The dosing space changes its volume nonlinearly when the bellows are first compressed slowly from the center thereof and then faster both from the center and the creases of the bellows. As the limited movement of the piezo-material is directly transferred to the bellows the repeatability of the droplet size dispensed is good but only a small number of droplets can be dispensed before the piezo-material reaches the end of its travel and the bellows needs to be re-filed.
U.S. Pat. No. 8,323,882 describes a syringe pump which is driven by a stepper motor and associated lead screw. Such systems can suffer from mechanical problems which affect their accuracy. For example play in the lead screw can lead to inaccurate dispensing where the drops that were intended to be dispensed at the beginning of a dispensing run are not actually dispensed as the system takes up the play in the system. Additionally it is possible that friction or static friction in the system prevents the stepper motor from actually completely a step (and thus fails to dispense any fluid). When the stepper motor is subsequently actuated to make the next step it is possible that it overcomes the friction and moves two steps instead of the newly commanded single step, thereby dispensing twice the intended amount of fluid. This is not a problem when large volumes are intended to be dispensed as these may involve tens or hundreds of continuous steps but in the case of small droplets requiring only one or a few steps this friction means that the repeatability of very small droplets sizes is often poor.
An example of a prior art bellows pump which attempts to overcome the disadvantages of earlier positive displacement pumps is known from WO2002/082024. This describes a bellows-pump in which the actuator is in the form of a voice coil comprising a magnet and a current coil. The magnet (alternatively the current coil) is attached to the movable end of the bellows and the current coil (alternatively the magnet) is attached to the body. The application of an electric current to the current coil produces a magnetic field which attracts or repulses the magnet and thereby changes the volume of the bellows. A decrease in the volume of the bellows forces a similar volume of liquid to leave the bellows via a dispensing nozzle. Such bellows pumps have low friction and thus do not suffer from the friction problems associated with syringe pumps.
FIG. 1 shows an example of a prior art positive displacement pump comprising a body 1 having a top cover 3, a bottom 5 and a cylindrical jacket 7 there between. A flexible bellows 9 is placed centrally in the space defined by the jacket 7. The bellows may be a nickel/cobalt bellows or fiber reinforced plastic bellows or the like. The lower end 10 of the bellows 9 is attached to an intermediate wall 11 which extends across the jacket and the opposite, movable end 13 of the bellows is arranged for linear movement in the axial directions of the bellows and the jacket 7 of the body. The movable end 13 of the bellows is moved by an actuator 17 formed of an axially extending rod 19 attached to a annular current coil 21 and a permanent magnet 23 attached to the bottom 5 of the body, and co-operating magnetically with the current coil. The current coil 21 is directly attached to the moving end 13 of the bellows and arranged to move in the axial direction of the jacket 7 in an annular groove 25 formed in the magnet 23 without contacting the magnet. Electric current may be passed to the current coil 21 through conductors 27, the magnitude of the current being adjusted by a control means such as a microprocessor 29 and drive circuitry. According to the direction of the current, the current coil 21 and the magnet 23 either attract or repel each other. The attraction or repulsion force is dependent on the strength of the current in coil 21. Normally, the current may be adjusted nearly continuously in the available adjusting range. To centralize the end of the bellows and the current coil 21 on the axis of the jacket 7 and to linearize the movements thereof, a centralizer 31 is placed between the jacket and the end of the bellows. The centralizer is represented schematically as a pleated, truncated conical spring. The widest end 33 of the spring is attached to the inner surface of the jacket 7 of the body and the truncated end 35 to the support 36 of the rod connected to movable end 13 of the bellows 9. The centralizer 31 has high lateral rigidity while yielding freely in the axial direction.
In the construction of the positive displacement pump, the spring force of the bellows 9 and the spring force of the centralizer 31 act against each other to balance the movable end 13 of the bellows at a zero position (X=0) in case where there is no electric current in the current coil 21. The movable end 13 of the bellows may thus move on both sides of the balance position according to the direction of the electric current passing through the coil 21.
A liquid chamber 37 filled with the liquid to be dosed is formed between the outside of the bellows 9, the intermediate wall, the jacket and the cover. The dosing operation carried out by the device is based on the volume changes of said liquid chamber. The liquid chamber 37 has a filling channel 39 provided with a pump 41 which can supply liquid from liquid supply 42 and can also act as a valve for closing the space. A dosing channel 43 leads from the liquid chamber to a dosing tip 45 discharging the dosed liquid amount in the form of droplets 47. The device presented may be used for producing test strips for chemical analyses onto a substrate 49 as shown in FIG. 1. Liquid droplets 47 can be injected on the strip 49 from the dosing tip 47. The positive displacement pump shown may be used in productive serial dosing wherein the liquid chamber 37 is filled with the liquid to be dosed, and thereafter, tens or even hundreds of small liquid doses having an equal volume are dosed by means of the actuator 17 constricting the liquid chamber to cause the dosing operations. In the initial position, when the bellows is retracted to its maximum extent, the magnet 23 and the current coil 21 attract each other against the total spring forces of the bellows 9 and the centralizer 15. Then, step by step, the magnitude of the current can be lowered to decrease the attracting forces of the magnet and the coil. This causes the coil and the end of the bellows to move axially away from the magnet, thus constricting the liquid chamber surrounding the bellows in a stepwise manner. Normally each step decrease in the current magnitude corresponds to the discharging of a predetermined amount of liquid 47 from the dosing tip 45. Once the magnitude of the electric current in the coil has reached the value of zero, said balance or zero position (X=0) of the movable end of the bellows is attained. Further dosing is achieved by reversing the direction of the electric current and by increasing the magnitude of the electric current step by step, the repelling force between the magnet and the coil thus pushing the movable end of the bellows against the total spring forces of the bellows and the centralizer and thereby constricting the liquid chamber further. In this manner, the serial dosing can be continued until the movable end of the bellows reaches the end of its practical range of movement +Xp and −Xp—which is normally set by the maximum current which the system can provide to the voice coil which corresponds to the maximum force which the coil can exert and is less than the theoretical maximum movements +Xt and −Xt which could be physically possible in the device.
A problem with such devices is that the working range of the voice coil is limited by the currents needed to move the current coil with respect to the magnet. This is because the current needed to displace the coil from its resting position (i.e. its position when there is no current flowing through it) does not vary linearly with distance but increases quadratically with the distance that the coil is from its resting position. This means that the actual working range of the pump, i.e. the volume that it is able to dispense before it needs refiling is much less than the theoretically-possible working range of the pump as it is limited by the current capacity of (and thus the power able to be generated by) the current coil/magnet actuator of the system. The further the voice coil is from its rest position, the higher the current needed to move it and/or maintain it in position. Apart from requiring a high power power-supply that is only used at its full capacity when the voice coil reaches the extremes of its range, the currents drawn by the voice coil generate heat in the system that can affect dispensing accuracy and stability. Furthermore the voice coil leads to inductive loads in the electrical circuits which have to be designed to withstand these loads.
A further drawback with a voice coil driven bellows pump is that it is not stiff in the axial dimension. Such an electromagnetic linear drive is dependent on continuous control of the current to achieve and maintain a desired position which requires expensive control circuitry and drivers.